dottxt-ai/outlines

Constrains LLM token generation to follow JSON schemas, Pydantic models, or custom grammars in real-time, eliminating post-hoc parsing failures. Integrates with any model backend—OpenAI, Ollama, vLLM, Hugging Face transformers—through a unified Python API while supporting multiple constraint engines (regex, context-free grammars, efficient FSM-based filtering). Designed for production workflows like customer support triage, e-commerce categorization, and function calling where guaranteed schema compliance is critical.

13,552 stars. Used by 12 other packages. Actively maintained with 23 commits in the last 30 days. Available on PyPI.